English summary: language markup in markup systems

The document
Kielimerkkaus: tekstin kielen ilmoittaminen merkkausjärjestelmissä
(in Finnish) discusses
language codes (including ISO 639 codes) in general but especially
language markup in markup systems such as
HTML and XML, partly in a fairly detailed manner, and emphasizing
the accessibility aspect, explaining the related WAI guidelines.
Although the principles are simple, there are complicated problems in
selecting appropriate markup especially at the phrase and word level,
not to mention parts of words.
For example, the abbreviation "USA" is clearly of English-language origin;
but e.g. in Finnish, it is read using either Finnish letter names or
as if it were a Finnish word.
Moreover, it is possible that a word in one language takes a suffix from
another language.
The author recommends that at word level,
markup be used to indicate language changes in unproblematic cases only;
"if in doubt, leave it out".
- As regards
documents on language markup in English, the author especially
recommends the nice introduction
Dan's Web Tips: Languages
and more detailed
Language Identifiers in the Markup Context.
For a deeper discussion, see the W3C working draft
Authoring Techniques for XHTML &
HTML Internationalization: Specifying the language of content.